Titanium-containing interference pigments and foils with color shifting properties

ABSTRACT

Interference pigment flakes and foils are provided which have color shifting properties. The pigment flakes can have a symmetrical coating structure on opposing sides of a reflector layer, can have an asymmetrical coating structure with all of the layers on one side of the reflector layer, or can be formed with encapsulating coatings around a core reflector layer. The coating structure of the flakes and foils includes a reflector layer, a dielectric layer on the reflector layer, and a titanium-containing absorber layer on the dielectric layer. The pigment flakes and foils exhibit a discrete color shift so as to have a first color at a first angle of incident light or viewing and a second color different from the first color at a second angle of incident light or viewing. The pigment flakes can be interspersed into liquid media such as paints or inks to produce colorant compositions for subsequent application to objects or papers. The foils can be laminated to various objects or can be formed on a carrier substrate.

CROSS-REFERENCE TO RELATED

This application is a divisional of U.S. patent application Ser. No.09/685,468, filed Oct. 10, 2000, now U.S. Pat. No. 6,569,529 andentitled “Titanium-Containing Interference Pigments And Foils With ColorShifting Properties” and claims the benefit thereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to interference pigments andfoils. More particularly, the present invention relates to multilayercolor shifting pigment flakes and foils which have titanium-containingabsorber layers.

2. Background Technology

Various color shifting pigments, colorants, and foils have beendeveloped for a wide variety of applications. For example, colorshifting pigments have been used in applications such as cosmetics,inks, coating materials, ornaments, ceramics, automobile paints,anti-counterfeiting hot stamps and anti-counterfeiting inks for securitydocuments and currency. Such pigments, colorants, and foils exhibit theproperty of changing color upon variation of the angle of incidentlight, or as the viewing angle of the observer is shifted.

The color shifting properties of the pigments and foils can becontrolled through proper design of the optical thin films used to formthe flake or foil coating structure. Desired effects can be achievedthrough the variation of parameters such as thickness of the layersforming the flakes and foils and the index of refraction of each layer.The changes in perceived color which occur for different viewing anglesor angles of incident light are a result of a combination of selectiveabsorption of the materials comprising the layers and wavelengthdependent interference effects. The interference effects, which arisefrom the superposition of light waves that have undergone multiplereflections, are responsible for the shifts in color perceived withdifferent angles. The reflection maxima changes in position andintensity, as the viewing angle changes, due to the absorptioncharacteristics of a material which are selectively enhanced atparticular wavelengths from the interference phenomena.

Various approaches have been used to achieve such color shiftingeffects. For example, small multilayer flakes, typically composed ofmultiple layers of thin films, are dispersed throughout a medium such aspaint or ink that may then be subsequently applied to the surface of anobject. Such flakes may optionally be overcoated to achieve desiredcolors and optical effects. Another approach is to encapsulate smallmetallic or silicatic substrates with varying layers and then dispersethe encapsulated substrates throughout a medium such as paint or ink.Additionally, foils composed of multiple layers of thin films on asubstrate material have been made.

One manner of producing a multilayer thin film structure is by formingit on a flexible web material with a release layer thereon. The variouslayers are deposited on the web by methods well known in the art offorming thin coating structures, such as PVD, sputtering, or the like.The multilayer thin film structure is then removed from the web materialas thin film color shifting flakes, which can be added to a polymericmedium such as various pigment vehicles for use as an ink or paint. Inaddition to the color shifting flakes, additives can be added to theinks or paints to obtain desired color shifting results.

Color shifting pigments or foils are formed from a multilayer thin filmstructure that includes the same basic layers. These include an absorberlayer(s), a dielectric layer(s), and optionally a reflector layer, invarying layer orders. The coatings can be formed to have a symmetricalmultilayer thin film structure, such as:

-   -   absorber/dielectric/reflector/dielectric/absorber; or        absorber/dielectric/absorber. Coatings can also be formed to        have an asymmetrical multilayer thin film structure, such as:    -   absorber/dielectric/reflector.

For example, U.S. Pat. No. 5,135,812 to Phillips et al. disclosesoptically variable thin film flakes having several differentconfigurations of layers such as transparent dielectric andsemi-transparent metallic layered stacks. In U.S. Pat. No. 5,278,590 toPhillips et al., incorporated by reference herein, a symmetric threelayer optical interference coating is disclosed which comprises firstand second partially transmitting absorber layers which have essentiallythe same composition and thickness, and a dielectric spacer layerlocated between the first and second absorber layers.

Color shifting platelets for use in paints are disclosed in U.S. Pat.No. 5,571,624 to Phillips et al., which is incorporated by referenceherein. These platelets are formed from a symmetrical multilayer thinfilm structure in which a first semi-opaque layer such as chromium isformed on a substrate, with a first dielectric layer formed on the firstsemi-opaque layer. An opaque reflecting metal layer such as aluminum isformed on the first dielectric layer, followed by a second dielectriclayer of the same material and thickness as the first dielectric layer.A second semi-opaque layer of the same material and thickness as thefirst semi-opaque layer is formed on the second dielectric layer.

Interference pigments having titanium dioxide layers have beenpreviously produced. For example, U.S. Pat. No. 5,116,664 to Kimura etal. discloses a pigment that is made by coating a first layer of TiO₂onto mica followed by coating the TiO₂ layer with powder particles of atleast one of the metals cobalt, nickel, copper, zinc, tin, gold, andsilver. The metallic powder layer is deposited by an electroless wetchemical process. Electron micrographs showed that these particles arein the form of finely divided rods.

Interference pigments incorporating titanium oxide layers are disclosedin U.S. Pat. Nos. 5,364,467 to Schmid et al. and 5,573,584 Ostertag etal. Each of these patents teaches colorless, non-absorbing TiO₂ layersor selectively absorbing metal oxide materials for overcoatingplatelet-like silicatic substrates (micas, talc or glass flakes) orplatelet-like metallic substrates.

U.S. Pat. No. 5,607,504 to Schmid et al. discloses pigments withtitanium(III) oxide, titanium oxynitride, and titanium nitride coatings,formed by the reduction of titanium dioxide. The pigment particles arecomposed of various metal substrates upon which is deposited aselectively absorbing coating of titanium oxynitrides and titaniumnitride with titanium dioxide and titanium III oxide by using hydrolyticdecomposition of titanium tetraisopropoxide or titanium tetrachlorideand subsequent reduction with ammonia.

In U.S. Pat. No. 4,978,394 to Ostertag, metal oxide coated aluminumpigments are disclosed, which include a substrate of platelet-likealuminum coated with layers of titanium oxides of different thicknesses.The titanium oxide layers are formed by a chemical vapor depositionprocess whereby titanium tetrachloride is reacted with water vapor.Optionally, the titanium dioxide layer can then be reduced to form TiO,TiN, or titanium oxynitrides through the use of H₂, CO, hydrocarbons orNH₃.

The electroless deposition methods and pyrolytic methods used inconventional techniques such as described above produce large islands ordots of material deposited on the substrate material. Hence, acontinuous coating is only obtained at the expense of depositing enoughcoating material to sufficiently coat the gaps between the islands ordots. This extensive deposition leads in turn to a relatively thickcoating which, because of its thickness, does not generate the bestchromatic colors.

Prior techniques for forming titanium-based coatings on a substrate havebeen limited to reducing an underlying titanium dioxide layer, resultingin a non-discrete layer interface. It is believed that the reduction ofTiO₂ layers results in high stress as the coating changes in chemicalstructure. As a result, voids may form in the coating if the newchemical structure requires less surface area and volume. Alternatively,bubbling may occur if the coating expands beyond its current surfacearea and volume because of a larger surface area being required toaccommodate the change in chemical structure. The structural flawsdegrade the optical qualities of the pigment.

Another difficulty with prior titanium coating techniques is thatfollowing the deposition of a titanium coating on a powdered substrate,the coated powder may auto-ignite by the spontaneous oxidization thatcan occur with a release of heat, as can happen during atmosphericventing of the vacuum chamber in a vacuum deposition process. Since thepowder particles are poor conductors of heat, the heat is trapped and arunaway oxidation occurs which consumes the entire powder mass.

Accordingly, there is a need for alternative absorber materials andcoating techniques which avoid the above drawbacks.

SUMMARY AND OBJECTS OF THE INVENTION

It is an object of the present invention to provide interference pigmentflakes and foils which exhibit color shifting properties.

It is another object of the invention to provide color shifting flakeswhich may be easily and economically utilized in colorants such aspaints and inks for various applications.

It is a further object of the invention to provide color shifting flakesand foils with environmentally safe absorbers.

Another object of the invention is to provide color shifting flakes thathave non-stick properties during fabrication.

A further object of the invention is to provide color shifting flakesthat are not prone to auto-ignition during fabrication.

An additional object of the invention is to provide a titanium-basedabsorber that can be deposited as a discrete uniform layer of easilyrepeatable thickness during fabrication of pigment flakes or foils.

A further object of the invention is to provide color shiftinginterference pigments and foils that exhibit high chroma and goodstability toward water, acid, base, bleach, and ultraviolet radiationexposure.

To achieve the forgoing objects and in accordance with the invention asembodied and broadly described herein, interference pigment flakes andfoils are provided which have color shifting properties. The pigmentflakes can have a symmetrical coating structure on opposing sides of areflector layer, can have an asymmetrical coating structure with all ofthe layers on one side of the reflector layer, or can be formed withencapsulating coatings around a core reflector layer. The coatingstructure of the flakes and foils includes a reflector layer, adielectric layer on the reflector layer, and a titanium-containingabsorber layer on the dielectric layer. The absorber layer can becomposed of elemental titanium, a titanium-based compound, or atitanium-based alloy. The titanium-containing absorber layer providesthe benefits of having benign chemical characteristics, as well asavoiding metal welding during the flake coating process. Titanium-basedabsorbers are also provided herein which avoid the auto-ignition problemof prior titanium coating techniques.

The pigment flakes and foils exhibit a discrete color shift so as tohave a first color at a first angle of incident light or viewing and asecond color different from the first color at a second angle ofincident light or viewing. The pigment flakes can be interspersed intoliquid media such as paints or inks to produce colorant compositions forsubsequent application to objects or papers. The foils can be laminatedto various objects or can be formed on a carrier substrate.

The foregoing objects and features of the present invention will becomemore fully apparent from the following description and appended claims,or may be learned by the practice of the invention as set forthhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to illustrate the manner in which the above-recited and otheradvantages and objects of the invention are obtained, a more particulardescription of the invention briefly described above will be rendered byreference to specific embodiments thereof which are illustrated in theappended drawings. Understanding that these drawings depict only typicalembodiments of the invention and are not therefore to be consideredlimiting of its scope, the invention will be described and explainedwith additional specificity and detail through the use of theaccompanying drawings in which:

FIG. 1 is a schematic representation of the coating structure of a colorshifting pigment flake according to one embodiment of the invention;

FIG. 2 is a cross-sectional schematic representation of the coatingstructure of a color shifting pigment flake according to anotherembodiment of the invention;

FIG. 3 is a cross-sectional schematic representation of the coatingstructure of a color shifting pigment flake according to an alternativeembodiment of the invention;

FIG. 4 is a cross-sectional schematic representation of the coatingstructure of a color shifting pigment flake according to a furtherembodiment of the invention;

FIG. 5 is a schematic representation of the coating structure of a colorshifting pigment flake according to another embodiment of the invention;

FIG. 6 is a schematic representation of the coating structure of a colorshifting pigment flake according to an additional embodiment of theinvention;

FIG. 7 is a cross-sectional schematic representation of the coatingstructure of a color shifting pigment flake according to a furtherembodiment of the invention;

FIG. 8 is a schematic representation of the coating sequence in formingan absorber layer of a color shifting pigment flake according to oneembodiment of the invention;

FIG. 9 is a schematic representation of the coating structure of a colorshifting foil according to one embodiment of the invention;

FIG. 10 is a schematic representation of the coating structure of acolor shifting foil according to another embodiment of the invention;

FIGS. 11 and 12 are alternative schematic configurations of the foil ofFIG. 9 formed on a web;

FIGS. 13 and 14 are graphs showing theoretical chromaticity plots basedon theoretical modeling of multilayer thin film structures;

FIG. 15 is a graph showing transmittance as a function of wavelength foran absorber layer of a color shifting film both before and afterextended water exposure;

FIG. 16 is a graph showing transmittance as a function of wavelength foran absorber layer of a color shifting film both before and afterextended water exposure;

FIGS. 17 and 18 are graphs showing reflectance as a function ofwavelength for the front and back sides of a color shifting film of theinvention.

FIGS. 19A and 19B make up an L*a*b* diagram which plots the colortrajectory and chromaticity of a color shifting foil of the invention;and

FIG. 20 is a chromaticity graph showing the measured color trajectoriesfor various panels painted with alternate color shifting pigments.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed generally to color shifting pigmentsand foils having titanium-containing absorber layers and methods ofmaking the same. The pigment flakes and foils have substantial shifts inchroma and hue with changes in angle of incident light or viewing angleof an observer. Such an optical effect, known as goniochromaticity,allows a perceived color to vary with the angle of illumination orobservation. Accordingly, the pigment flakes and foils exhibit a firstcolor at a first angle of incident light or viewing and a second colordifferent from the first color at a second angle of incident light orviewing. The pigment flakes can be interspersed into liquid media suchas paints or inks to produce various color shifting colorantcompositions for subsequent application to objects or papers. The foilscan be laminated to various objects or can be formed on a carriersubstrate.

Generally, the pigment flakes can have a symmetrical coating structureon opposing sides of a reflector layer, can have an asymmetrical coatingstructure with all of the layers on one side of the reflector layer, orcan be formed with encapsulating coatings which surround a reflectorlayer core. The coating structure of the flakes and foils generallyincludes a reflector layer, a dielectric layer on the reflector layer,and a titanium-containing absorber layer on the dielectric layer. Eachof these layers in the coating structures of the flakes and foils of theinvention will be discussed in further detail hereinafter. It is afeature of the invention that as least one absorber layer of the pigmentor foil is a titanium or titanium-based absorber that is deposited as adiscrete, outermost layer of the pigment flake or foil.

The color shifting flakes and foils of the invention can be formed usingconventional thin film deposition techniques, which are well known inthe art of forming thin coating structures. Nonlimiting examples of suchthin film deposition techniques include physical vapor deposition (PVD),chemical vapor deposition (CVD), plasma enhanced (PE) variations thereofsuch as PECVD or downstream PECVD, sputtering, electrolysis deposition,and other like deposition methods that lead to the formation of discreteand uniform thin film layers.

The color shifting pigment flakes of the invention can be formed byvarious fabrication methods. For example, the pigment flakes can beformed by a web coating process in which various layers are sequentiallydeposited on a web material by conventional deposition techniques toform a thin film structure, which is subsequently fractured and removedfrom the web, such as by use of a solvent, to form a plurality of thinfilm flakes.

In another fabrication method, one or more thin film layers including atleast the reflector layer is deposited on a web to form a film, which issubsequently fractured and removed from the web to form a plurality ofpigment preflakes. The preflakes can be fragmented further by grindingif desired. The preflakes are then coated with the remaining layer orlayers in a sequential encapsulation process to form a plurality ofpigment flakes. Such a process is disclosed in further detail incopending U.S. application Ser. No. 09/512,116, filed on Feb. 24, 2000,the disclosure of which is incorporated by reference herein.

In another alternative fabrication method, reflective particles can becoated in a sequential encapsulation process to form a plurality ofpigment flakes. When an encapsulation process is used for forming theouter layers of the flakes, it will be appreciated that each respectiveencapsulating layer is a continuous layer composed of one material andhaving substantially the same thickness around the flake structure.

Referring to the drawings, wherein like structures are provided withlike reference designations, the drawings only show the structuresnecessary to understand the present invention. FIG. 1 depicts a colorshifting pigment flake 10 with titanium-containing absorber layersaccording to one embodiment of the invention. The flake 10 is afive-layer design having a generally symmetrical multilayer thin filmstructure on opposing sides of a reflector layer 12. Thus, first andsecond dielectric layers 14 and 15 are disposed respectively on opposingsides of reflector layer 12, and first and second titanium-basedabsorber layers 18 and 19 are disposed respectively on each ofdielectric layers 14 and 15. Each of these layers in the coatingstructure of flake 10 is discussed below in greater detail.

The reflector layer 12 can be composed of various materials. Presentlypreferred materials are one or more metals, one or more metal alloys, orcombinations thereof, because of their high reflectivity and ease ofuse, although non-metallic reflective materials could also be used.Nonlimiting examples of suitable metallic materials for reflector layer12 include aluminum, silver, copper, gold, platinum, tin, titanium,palladium, nickel, cobalt, rhodium, niobium, chromium, and combinationsor alloys thereof. These can be selected based on the color effectsdesired. The reflector layer 12 can be formed to have a suitablephysical thickness of from about 200 angstroms (Å) to about 1000 Å, andpreferably from about 400 Å to about 700 Å.

The dielectric layers 14 and 15 act as spacers in the thin film stackstructure of flake 10. These layers are formed to have an effectiveoptical thickness for imparting interference color and desired colorshifting properties. The dielectric layers may be optionally clear, ormay be selectively absorbing so as to contribute to the color effect ofa pigment. The optical thickness is a well known optical parameterdefined as the product ηd, where η is the refractive index of the layerand d is the physical thickness of the layer. Typically, the opticalthickness of a layer is expressed in terms of a quarter wave opticalthickness (QWOT) that is equal to 4ηd/λ, where λ is the wavelength atwhich a QWOT condition occurs. The optical thickness of dielectriclayers can range from about 2 QWOT at a design wavelength of about 400nm to about 9 QWOT at a design wavelength of about 700 nm, andpreferably 2-6 QWOT at 400-700 nm, depending upon the color shiftdesired. The dielectric layers typically have a physical thickness ofabout 100 nm to about 800 nm.

Suitable materials for dielectric layers 14 and 15 include those havinga “high” index of refraction, defined herein as greater than about 1.65,as well as those have a “low” index of refraction, which is definedherein as about 1.65 or less. Each of the dielectric layers can beformed of a single material or with a variety of material combinationsand configurations. For example, the dielectric layers can be formed ofonly a low index material or only a high index material, a mixture ormultiple sublayers of two or more low index materials, a mixture ormultiple sublayers of two or more high index materials, or a mixture ormultiple sublayers of low index and high index materials. In addition,the dielectric layers can be formed partially or entirely of high/lowdielectric optical stacks, which are discussed in further detail below.When a dielectric layer is formed partially with a dielectric opticalstack, the remaining portion of the dielectric layer can be formed witha single material or various material combinations and configurations asdescribed above.

Examples of suitable high refractive index materials for the dielectriclayer include zinc sulfide (ZnS), zinc oxide (ZnO), zirconium oxide(ZrO₂), titanium dioxide (TiO₂), carbon (C), indium oxide (In₂O₃),indium-tin-oxide (ITO), tantalum pentoxide (Ta2O5), ceric oxide (CeO₂),yttrium oxide (Y₂O₃), europium oxide (Eu₂O₃), iron oxides such as(II)diiron(III) oxide (Fe₃O₄) and ferric oxide (Fe₂O₃), hafnium nitride(HfN), hafnium carbide (HfC), hafnium oxide (HfO₂), lanthanum oxide(La₂O₃), magnesium oxide (MgO), neodymium oxide (Nd₂O₃), praseodymiumoxide (Pr₆O₁₁), samarium oxide (Sm₂O₃), antimony trioxide (Sb₂O₃),silicon carbide (SiC), silicon nitride (Si₃N₄), silicon monoxide (SiO),selenium trioxide (Se₂O₃), tin oxide (SnO₂), tungsten trioxide (WO₃),combinations thereof, and the like.

Suitable low refractive index materials for the dielectric layer includesilicon dioxide (SiO₂), aluminum oxide (Al₂O₃), metal fluorides such asmagnesium fluoride (MgF₂), aluminum fluoride (AlF₃), cerium fluoride(CeF₃), lanthanum fluoride (LaF₃), sodium aluminum fluorides (e.g.,Na₃AlF₆ or Na₅Al₃F₁₄), neodymium fluoride (NdF₃), samarium fluoride(SmF₃), barium fluoride (BaF₂), calcium fluoride (CaF₂), lithiumfluoride (LiF), combinations thereof, or any other low index materialhaving an index of refraction of about 1.65 or less. For example,organic monomers and polymers can be utilized as low index materials,including dienes or alkenes such as acrylates (e.g., methacrylate),perfluoroalkenes, polytetrafluoroethylene (Teflon), fluorinated ethylenepropylene (FEP), combinations thereof, and the like.

It should be appreciated that several of the above-listed dielectricmaterials are typically present in non-stoichiometric forms, oftendepending upon the specific method used to deposit the dielectricmaterial as a coating layer, and that the above-listed compound namesindicate the approximate stoichiometry. For example, silicon monoxideand silicon dioxide have nominal 1:1 and 1:2 silicon:oxygen ratios,respectively, but the actual silicon:oxygen ratio of a particulardielectric coating layer varies somewhat from these nominal values. Suchnon-stoichiometric dielectric materials are also within the scope of thepresent invention.

As mentioned above, the dielectric layers can be formed of high/lowdielectric optical stacks, which have alternating layers of low index(L) and high index (H) materials. When a dielectric layer is formed of ahigh/low dielectric stack, the color shift at angle will depend on thecombined refractive index of the layers in the stack. Examples ofsuitable stack configurations for the dielectric layers include LH, HL,LHL, HLH, HLHL, LHLH, as well as various multiples and combinationsthereof. In these stacks, LH, for example, indicates discrete layers ofa low index material and a high index material. In an alternativeembodiment, the high/low dielectric stacks are formed with a gradientindex of refraction. For example, the stack can be formed with layershaving a graded index low-to-high, a graded index high-to-low, a gradedindex low-to-high-to-low, a graded index high-to-low-to-high, as well ascombinations and multiples thereof. The graded index is produced by agradual variance in the refractive index, such as low-to-high index orhigh-to-low index, of adjacent layers. The graded index of the layerscan be produced by changing gases during deposition or co-depositing twomaterials (e.g., L and H) in differing proportions. Various high/lowoptical stacks can be used to enhance color shifting performance,provide antireflective properties to the dielectric layer, and changethe possible color space of the pigments of the invention.

The dielectric layers can each be composed of the same material or adifferent material, and can have the same or different optical orphysical thickness for each layer. It will be appreciated that when thedielectric layers are composed of different materials or have differentthicknesses, the flakes exhibit different colors on each side thereofand the resulting mix of flakes in a pigment or paint mixture would showa new color which is the combination of the two colors. The resultingcolor would be based on additive color theory of the two colors comingfrom the two sides of the flakes. In a multiplicity of flakes, theresulting color would be the additive sum of the two colors resultingfrom the random distribution of flakes having different sides orientedtoward the observer.

The titanium-based absorber layers 18 and 19 can be composed of anabsorbing material, including selective and non-selective absorbingmaterials, having the desired absorption properties. The absorbingmaterial is preferably substantially free of titanium dioxide. The terms“substantially free” as used herein means that while TiO₂ is notintentionally formed in the absorber layers, trace amounts of TiO₂ canexist in the absorber layers because of the high reactivity of Ti withO₂. The titanium-containing absorber layers used in the presentinvention are non-toxic, durable, and maintain good optical propertiessuch as high chroma.

The absorber layers 18 and 19 can be composed of elemental titanium,titanium-based alloys, titanium-based compounds, or mixtures thereof.Examples of suitable titanium-based alloys include titanium mixed withcarbon (Ti/C), titanium mixed with tungsten (Ti/W), titanium mixed withniobium (Ti/Nb), titanium mixed with silicon (Ti/Si), and combinationsthereof. Examples of suitable titanium-based compounds include titaniumnitride (TiN), titanium oxynitride (TiN_(x)O_(y)), titanium carbide(TiC), titanium nitride carbide (TiN_(x)C_(z)), titanium oxynitridecarbide (TiN_(x)O_(y)C_(z)), titanium silicide (TiSi₂), titaniumdiboride (TiB₂), and combinations thereof. In the case of TiN_(x)O_(y)and TiN_(x)O_(y)C_(z), preferably x=0 to 1, y=0 to 1, and z=0 to 1,where x+y=1 in TiN_(x)O_(y) and x+y+z=1 in TiN_(x)O_(y)C_(z). ForTiN_(x)C_(z), preferably x=0 to 1 and z=0 to 1, where x+z=1. Theabsorber layers may also be composed of various combinations of theabove absorbing materials. For example, an absorber layer can becomposed of a titanium-based alloy disposed in a matrix of Ti, or can becomposed of Ti disposed in a matrix of a titanium-based alloy.

The titanium-containing absorber layers can be formed to have a suitablephysical thickness of from about 30 Å to about 300 Å, preferably fromabout 100 Å to about 175 Å. The absorber layers can each be composed ofthe same titanium-based material or different titanium-based materials,and can have the same or different physical thickness for each layer.The titanium-containing absorber layers are preferably non-toxic,durable and maintain good optical properties as absorber layers such ashigh chroma.

One presently preferred method of fabricating a plurality of pigmentflakes, each of which have the multilayer thin film coating structure offlake 10, is based on conventional web coating techniques used to makeoptical thin films. Accordingly, a first titanium-containing absorberlayer is deposited on a web of flexible material such as polyethyleneterephthalate (PET) which has an optional release layer thereon. Theabsorber layer can be formed by a conventional deposition process suchas PVD, CVD, PECVD, sputtering, or the like. The above mentioneddeposition methods enable the formation of a discrete and uniformabsorber layer of a desired thickness.

Next, a first dielectric layer is deposited on the titanium-containingabsorber layer to a desired optical thickness by a conventionaldeposition process. The deposition of the dielectric layer can beaccomplished by a vapor deposition process (e.g., PVD, CVD, PECVD),which results in the dielectric layer cracking under the stressesimposed as the dielectric transitions from the vapor into the solidphase.

The reflector layer is then deposited on the first dielectric layer,taking on the characteristics of the underlying cracked dielectriclayer. This is followed by a second dielectric layer being deposited onthe reflector layer and preferably having the same optical thickness asthe first dielectric layer. Finally, a second titanium-containingabsorber layer is deposited on the second dielectric layer andpreferably has the same physical thickness as the first absorber layer.

Thereafter, the flexible web is removed, either by dissolution in apreselected liquid or by way of a release layer, both of which are wellknown to those skilled in the art. As a result, a plurality of flakesare fractured out along the cracks of the layers during removal of theweb from the multilayer thin film. This method of manufacturing pigmentflakes is similar to that more fully described in U.S. Pat. No.5,135,812 to Phillips et al., the disclosure of which is incorporated byreference herein. The pigment flakes can be further fragmented ifdesired by, for example, grinding the flakes to a desired size using anair grind, such that each of the pigment flakes has a dimension on anysurface thereof ranging from about 2 microns to about 200 microns.

In an alternative embodiment of flake 10, an asymmetrical color shiftingflake can be provided which includes a three-layer thin film stackstructure with the same layers as on one side of the reflector layer inflake 10 as shown in FIG. 1. Accordingly, the asymmetrical colorshifting flake includes reflector layer 12, dielectric layer 14 onreflector 12, and absorber layer 18 on dielectric layer 14. Each ofthese layers can be composed of the same materials and have the samethicknesses as described above for the corresponding layers of flake 10.In addition, asymmetrical color shifting flakes can be formed by a webcoating process such as described above in which the various layers aresequentially deposited on a web material to form a thin film structure,which is subsequently fractured and removed from the web to form aplurality of flakes.

FIG. 2 depicts alternative coating structures (with phantom lines) for acolor shifting pigment flake 20 in the form of an encapsulate accordingto other embodiments of the invention. The flake 20 has a core reflectorlayer 22, which can be overcoated by an encapsulating dielectric layer24 substantially surrounding the reflector layer 22. A titanium-basedabsorber layer 26, which overcoats dielectric layer 24, provides anouter encapsulation of flake 20. The hemispherical lines on one side offlake 20 in FIG. 2 indicate that dielectric layer 24 and titanium-basedabsorber layer 26 can be formed as contiguous layers.

Alternatively, the reflector layer and dielectric layer can be in theform of a thin film core flake stack, in which opposing dielectriclayers 24 a and 24 b are preformed on the top and bottom surfaces butnot on at least one side surface of reflector layer 22, with absorberlayer 26 encapsulating the thin film stack. An encapsulation process canalso be used to form additional layers on flake 20 such as a cappinglayer (not shown). Suitable materials and thicknesses for the absorber,dielectric, and reflector layers of flake 20 are the same as taughthereinabove for flake 10.

In addition, core reflector layer 22 can be a multi-layered core flakesection structure, such as a “bright metal flake” as disclosed in U.S.Pat. No. 6,013,370 to Coulter et al., and U.S. application Ser. No.09/207,121, filed Dec. 7, 1998, the disclosures of which areincorporated by reference herein. Such a multi-layered structureincludes a reflector sublayer having a top surface, a bottom surface,and at least one side surface, and a support sublayer preformed on atleast one of the top and bottom surfaces but not on the at least oneside surface of the reflector sublayer. The reflector sublayer can be ametal such as aluminum having a thickness of at least about 40 nm, andthe support layer(s) can be a dielectric such as silicon oxide having athickness of at least about 10 nm, with the thickness being chosen sothat the dielectric sublayers do not substantially affect the colorproperties of the reflector sublayer. For example, a multilayered coreflake section can have the coating structure SiO_(x)/Al/SiO_(x), where xis from about 1 to about 2.

The core reflector layer 22 can also be a multi-layered structure suchas a “composite reflective flake” as disclosed in copending U.S.application Ser. No. 09/626,041 to Coulter et al., filed Jul. 27, 2000,the disclosure of which is incorporated by reference herein. Such amulti-layered structure includes a central support sublayer having a topsurface, a bottom surface, and at least one side surface, and areflector sublayer preformed on one or both of the top and bottomsurfaces but not on the at least one side surface of the reflectorsublayer.

FIG. 3 depicts another alternative coating structure for a colorshifting pigment flake 21 according to the present invention. The flake21 includes a core reflector layer 22 and a single dielectric layer 24,which extends over top and bottom surfaces of reflector layer 22 to forma dielectric-coated preflake 23. The dielectric-coated preflake 23 hastwo side surfaces 25 and 27. Although side surface 25 is homogeneous andformed only of the dielectric material of dielectric layer 24, sidesurface 27 has distinct surface regions 27 a, 27 b, 27 c of dielectric,reflector, and dielectric, respectively. The dielectric-coated preflake23 is further coated on all sides with a titanium-containing absorberlayer 26. The absorber layer 26 is in contact with dielectric layer 24and reflector layer 22 at side surface 27.

The structure of pigment flake 21 typically occurs because of a preflakecoating process such as disclosed in U.S. application Ser. No.09/512,116 described previously. As described therein, the preflakes canbe a dielectric-coated flake, in which a dielectric coating completelyencapsulates a core flake section. The preflakes are broken into sizedpreflakes using any conventional fragmentation process, such as bygrinding. The sized preflakes will include some sized preflakes havingtop and bottom dielectric layers with no dielectric coating on the sidesurfaces of the preflake, such as shown for the embodiment of flake 20in FIG. 2 in which reflector layer 22 is coated with top and bottomdielectric layers 24 a and 24 b. Other sized preflakes will have asingle dielectric layer extending over both top and bottom surfaces ofthe core flake section, leaving one side surface of the core flakesection exposed, such as shown for dielectric-coated preflake 23 in FIG.3. Because of the fragmentation process, substantially all of the sizedpreflakes have at least a portion of a side surface exposed. The sizedpreflakes are then coated on all sides with a titanium-containingabsorber layer, such as shown in the flakes of FIGS. 2 and 3.

FIG. 4 depicts another alternative coating structure for a colorshifting pigment flake 28 in the form of an encapsulate. The flake 28has a thin core layer 29, which can be formed of a particulate substratematerial that provides rigidity, such as mica, glass flake, talc, orother silicatic material, as well as iron oxide, boron nitride, and thelike. The core layer 29 is overcoated on all sides with a reflectorcoating 31, such as a reflective metallic coating, which can be composedof the same materials as described above for reflector layer 12 of flake10. An encapsulating dielectric layer 24 substantially surrounds corelayer 29 and reflector coating 31. A titanium-based absorber layer 26,which overcoats dielectric layer 24, provides an outer encapsulation offlake 28.

Various coating processes can be utilized in forming the dielectric andabsorber coating layers by encapsulation. For example, suitablepreferred methods for forming the dielectric layer include vacuum vapordeposition, sol-gel hydrolysis, CVD in a fluidized bed, andelectrochemical deposition. A suitable SiO₂ sol-gel process is describedin U.S. Pat. No. 5,858,078 to Andes et al., the disclosure of which isincorporated by reference herein. Other examples of suitable sol-gelcoating techniques useful in the present invention are disclosed in U.S.Pat. No. 4,756,771 to Brodalla; Zink et al., Optical Probes andProperties of Aluminosilicate Glasses Prepared by the Sol-Gel Method,Polym. Mater. Sci. Eng., 61, pp. 204-208 (1989); and McKieman et al.,Luminescence and Laser Action of Coumarin Dyes Doped in Silicate andAluminosilicate Glasses Prepared by the Sol-Gel Technique, J. Inorg.Organomet. Polym., 1(1), pp. 87-103 (1991); with the disclosures of eachof these incorporated by reference herein.

Suitable preferred methods for forming the absorber layers includevacuum vapor deposition, and sputtering onto a mechanically vibratingbed of particles, as disclosed in commonly assigned copending patentapplication Ser. No. 09/389,962, filed Sep. 3, 1999, entitled “Methodsand Apparatus for Producing Enhanced Interference Pigments,” which isincorporated by reference herein in its entirety. Alternatively, theabsorber coating may be deposited by decomposition through pyrolysis ofmetal-organo compounds or related CVD processes which may be carried outin a fluidized bed as described in U.S. Pat. Nos. 5,364,467 and5,763,086 to Schmid et al., the disclosures of which are incorporated byreference herein. If no further grinding is carried out, these methodsresult in an encapsulated core flake section with dielectric andabsorber materials therearound. Various combinations of the abovecoating processes may be utilized during manufacture of pigment flakeswith multiple encapsulating coatings.

In one preferred method, a sol-gel process is used as part of thefabrication method in producing coated powdered flakes of the invention,each of which have the coating structure of flake 20. The core reflectorlayer 22 in such flakes is a bright metal flake (BMF), which in oneembodiment is deposited on a roll coater release layer asSiOx/Al/SiO_(x) and removed as flakes by dissolving the release layer.An encapsulating dielectric layer 24 is then formed around the flakes bya sol-gel process, which in one embodiment produces a sol-gel SiO₂coating. This is effected by placing the flakes in a stirred reactorwith isopropyl alcohol (IPA), water, ammonia, and tetraethoxysilane(TEOS). The TEOS is reactively decomposed to form SiO₂ onto the surfacesof the SiO_(x) of the BMF.

This sol-gel based powdered flake material is then dried and fed into aparticle vacuum coating chamber to be coated with a titanium-basedabsorber layer 26 on each flake, such as a titanium/carbon mixture,titanium carbide, titanium nitride, titanium nitride carbide, or amixture of titanium plus the aforementioned materials. In the case oftitanium/carbon, the deposition on the flakes can occur in a sputteringprocess, such as direct magnetron sputtering, by running more than onetarget where one or more targets are set to sputter titanium and theother target(s) are set to sputter carbon. Alternatively, a reactivesputtering process can be utilized to deposit titanium nitride, titaniumcarbide, or titanium nitride carbide layers by running a target set tosputter titanium, along with nitrogen and/or methane gases (e.g.,Ti+N₂+CH₄ →TiN_(x)C_(z)). An in-situ color monitor can be used toindicate the highest level of chroma on the coated particles in order todetermine coating time. The coating chamber is then cooled and carefullybackfilled with atmospheric air until the chamber reaches atmosphericpressure.

In one method of forming the titanium-containing coating, the sol-gelpowdered flakes or other coated preflakes are placed on a square-shapedvibrating conveyor coater in a vacuum coating chamber as disclosed inU.S. application Ser. No. 09/389,962, discussed above. The vibratingconveyor coater includes conveyor trays which are configured in anoverlapping inclined arrangement so that the powdered flakes travelalong a circulating path within the vacuum chamber. While the flakescirculate along this path they are effectively mixed by constantagitation so that exposure to the vaporized absorber coating material isuniform. Efficient mixing also occurs at the end of each conveyor trayas the flakes drop in a waterfall off of one tray and onto the nexttray. The absorber can be sequentially coated on the flakes as theyrepeatably move under a coating material source.

When using vibrating conveyer trays to coat the absorber, it isimportant that the powdered flakes tumble randomly under the coatingmaterial source such as sputter targets and do not become subject to“metal welding” or sticking. Such metal welding or sticking can occurbetween two flat surfaces of reactive metals when such metals aredeposited in a vacuum. For example, aluminum has a high propensity tostick to itself, whereas chromium does not. It has been discovered thatthe titanium-based absorber coatings of the present invention providelubricity to the flake particles so that metal welding or sticking ofthe newly coated surfaces does not occur, allowing for the flow of flakeparticles against one another as they travel along the vibrating trays.Preferred titanium-based absorber materials which provide suitablelubricity and allow for good flowability include a titanium/carbonmixture, titanium carbide, and titanium nitride (e.g., titaniumsputtered reactively with nitrogen). Such absorber materials can beapplied as either a single material or as an outer capping layer over anunderlying different absorber material. Titanium nitride may be also bemixed with titanium in such an amount so as to allow good flowabilityand avoid auto-ignition upon exposing the pigment flakes to atmosphericpressure.

In addition, TiO₂ or other metal oxides can be added as an outer cappinglayer over an underlying Ti-based absorber layer in the pigment flakesof the invention. The deposition of a capping layer in vacuum aids inavoiding the auto-ignition problem of prior titanium coating techniques,since the capping layer helps to stabilize the pigment flake productwhen brought into the atmosphere.

Another method of depositing the titanium-containing absorbers of theinvention is by plasma enhanced chemical vapor deposition (PECVD) wherethe chemical species are activated by a plasma. In PECVD processes, thinfilm deposition occurs at lower temperatures than can be achieved withother CVD reactors, without sacrificing quality. The PECVD processes useelectrical energy to transform a gas mixture into a plasma containingreactive radicals, ions, neutral atoms and molecules, as well as otherreactive species. Because the reactions occur by collisional processesin the gas phase, the temperature can be kept lower than in typical CVDprocesses. Different techniques can be used to apply a plasma, the mostcommon being radio frequency (RF) and microwave (MW) plasma discharges.Also, depending on the substrate position, the PECVD process can becategorized as direct PECVD or downstream PECVD (DsPECVD).

In the case of direct PECVD, all the chemical species in the gas phaseare plasma activated. In DsPECVD, the process gases and the reactant orprecursors gases can be separated, the process gases being exposed tothe direct plasma and the precursors or reactant gases introduceddownstream in the plasma. Although both forms of PECVD can be utilized,the DsPECVD configuration is advantageous because there is no depositionof the film on the dielectric walls of the plasma applicator orcontamination of the applicators from the powdered substrate.

For DsPECVD, it is desirable to have a high-density plasma source (e.g.,ion densities greater than 10¹¹ cm³). Typically, inductive coupling ofRF power (0.5 to 100 MHz) can produce ion densities in excess of 10¹²cm³. In the case of a downstream plasma, the inductive circuit elementis adjacent (outside a dielectric wall) to the discharge region in orderto couple energy from the RF power source to an ionized gas. Dependingon the configuration, RF power source couplers can be categorized ashelical couplers, helical resonators, spiral couplers, and transformercouplers.

High-density plasmas can also be obtained using microwave discharges.Typically, for microwave plasma generation outside a reactor,rectangular wave-guides guide the microwaves from a generator to aplasma applicator. The plasma generated in this way inside a dielectricchamber is sufficient for the effective activation of process andreactive gases introduced directly in the plasma (direct PECVD) ordownstream (DsPECVD).

In the case of titanium nitride, titanium carbide, or titanium nitridecarbide coatings formed by a PECVD process, a titanium-containing gas isreacted with a working gas such as nitrogen (forming titanium nitride)or a hydrocarbon gas such as methane (forming titanium carbide) or both(forming titanium nitride carbide) and deposited on the flakes. Suitablesources for the titanium-containing gas include titanium halides such astitanium tetrachloride (TiCl₄), and metallo-organic titanium compounds.It should be noted that when TiN, TiC, or TiN_(x)C_(z) absorbers aredeposited by a PECVD process, these compounds can be considered asTiN_(x)O_(y), TiC_(x)O_(y), and TiN_(x)O_(y)C_(z), respectively, becauseof oxygen contamination in the plasma reactors and the high reactivityof titanium to oxygen. The PECVD processes result in good coatingadhesion, low pinhole density, good step coverage, and coatinguniformity.

FIG. 5 depicts a color shifting pigment flake 30 according to anotherembodiment of the invention. The flake 30 is a nine-layer design havinga generally symmetrical multilayer thin film stack structure on opposingsides of a reflector layer 32. Thus, first and second dielectric layers34 a and 34 b are disposed respectively on opposing sides of reflectorlayer 32, and first and second titanium-based absorber layers 36 a and36 b are disposed respectively on each of dielectric layers 34 a and 34b. A third dielectric layer 38 a is formed on absorber layer 36 a, and afourth dielectric layer 38 b is formed on absorber layer 36 b. A thirdtitanium-based absorber layer 40 a is on dielectric layer 38 a, and afourth titanium-based absorber layer 40 b is on dielectric layer 38 b.These layers of flake 30 can be formed by a web coating and flakeremoval process as described previously.

As shown in FIG. 5, each dielectric and absorber layer pair forms arepeating period 42, 44 of dielectric/absorber (e.g., layers 34 a and 36a, and layers 38 a and 40 a). One or more additional periods ofdielectric/absorber layers may be added to flake 30 to obtain a desiredoptical effect.

FIG. 5 further shows an alternative coating structure (with phantomlines) for color shifting flake 30, in which one or more of the absorberlayers and dielectric layers are coated around reflector layer 32 in anencapsulation process. For example, when an encapsulation process isused for the outer absorber layer, absorber layers 40 a and 40 b areformed as part of a continuous coating layer 40 substantiallysurrounding the flake structure thereunder. Likewise, an encapsulationprocess can also be used in forming the underlying dielectric layer,such that dielectric layers 38 a and 38 b are formed as part of acontinuous coating layer 38 substantially surrounding the flakestructure thereunder. An encapsulation process can also be used informing the other dielectric and absorber layers 34 and 36, such thatreflector layer 32 is encapsulated sequentially with alternatingdielectric and absorber layers.

Thus, pigment flake 30 may be embodied either as a multilayer thin filmstack flake or a multilayer thin film encapsulated particle with one ormore encapsulating layers therearound. Suitable materials andthicknesses for the absorber, dielectric, and reflector layers of flake30 are the same as taught hereinabove for flake 10.

FIG. 6 depicts a color shifting pigment flake 50 according to anotherembodiment of the invention which does not use a reflector. The flake 50is a three-layer design having a generally symmetrical multilayer thinfilm structure on opposing sides of a dielectric layer. Thus, first andsecond titanium-based absorber layers 54 a and 54 b are formed onopposing major surfaces of dielectric layer 52. These layers of flake 50can be formed by a web coating and flake removal process as describedpreviously.

FIG. 6 further depicts an alternative coating structure (with phantomlines) for color shifting flake 50, in which the absorber layer iscoated around dielectric layer 52 in an encapsulation process.Accordingly, absorber layers 54 a and 54 b are formed as part of acontinuous coating layer 54 substantially surrounding the flakestructure thereunder.

Thus, pigment flake 50 may be embodied either as a multilayer thin filmstack flake or a multilayer thin film encapsulated particle. Suitablematerials and thicknesses for the absorber and dielectric layers offlake 50 are the same as taught hereinabove for flake 10.

FIG. 7 illustrates a pigment flake 60 according to a further embodimentof the present invention. Pigment flake 60 comprises a core layer 62which is substantially encapsulated by a first titanium-based absorberlayer 64 by a coating process, such as by use of a vibrating tray systemas described previously. The absorber layer 64 is in turn encapsulatedby a silicon dioxide layer 66 formed by a sol-gel process. A secondtitanium-based absorber layer 68 encapsulates silicon dioxide layer 66.Thus, pigment flake 60 is embodied as a multilayer thin filmencapsulated particle. The core layer 62 is preferably a flat,transparent planar material such as mica, glass, or other likedielectric material, which gives strength to the flake. Suitablematerials and thicknesses for the absorber layers of flake 60 are thesame as taught hereinabove for flake 10. A high index TiO₂ sol-gel basedlayer may be substituted for the SiO₂ sol-gel layer in flake 60 toprovide a pigment with a smaller color shift.

The coating methods utilized in the present invention to form the abovedescribed embodiments result in the titanium-containing absorber layerbeing formed on the underlying dielectric layer such that the absorberlayer has a discrete boundary interface with the underlying dielectriclayer. The coating methods used in the invention also result in discreteuniform absorber layers that are more easily repeatable.

The present coating methods for depositing titanium-based absorbers arealso advantageous over conventional non-deposition techniques. Forexample, the conventional method of obtaining titanium-based layers bythe reduction of TiO₂ to TiN_(x) in ammonia will result in a crystallinestructure of non-uniform thickness with no discrete, sharp interlayerinterface. In contrast, the present deposition methods taught hereinresult in the titanium-containing absorber layer being composed of anamorphous absorbing material with substantially no crystallinestructure, having a well defined interlayer interface.

For example, FIG. 8 is a schematic representation of the coatingsequence in forming a titanium-containing absorber layer of a colorshifting pigment flake 70 according to one embodiment of the invention.The pigment flake 70 has a preflake structure 71, which includes areflector layer 72 and dielectric layers 74 a and 74 b on opposing sidesof reflector layer 72. A titanium-based absorber layer 76 is formed bydepositing alternating sublayers 80 a/82 a through 80 n/82 n of titaniumand carbon such as by a sputtering process in which preflake 71 isrepeatably moved passed a titanium target and a carbon target. This canbe accomplished by use of the vibrating conveyer trays in a vacuumchamber as discussed previously. Each of the titanium and carbonsublayers can be deposited at atomic layer thicknesses, resulting in thesublayers having no discrete interfaces within absorber layer 76. Thetitanium and carbon sublayers thus form an intimate mixture at theatomic level of these two elements, effectively producing an amorphousabsorber layer. For example, if the final thickness of the absorberlayer is 150 Å, and the number of passes by a target is 440 (e.g., 110minutes of coating time and 15 seconds to make the circular trip in thetrays), then each sublayer is deposited at a thickness of about 0.3Å(150/440). Alternatively, if each sublayer is deposited at greater thanatomic layer thicknesses, then the titanium and carbon sublayers canhave discrete interfaces with adjacent sublayers within absorber layer76, or islands of titanium and carbon interspersed together can beformed, depending on the deposition rate utilized.

Similarly, a TiN absorber layer can be formed as an amorphous layerwithout crystalline structure. For example, a coating time of 110minutes, with 15 seconds to make the circular trip in the trays, for a250 g charge of one titanium target in the presence of N₂ gas to formTiN with the maximum chroma results in about 440 separate coatingdepositions onto the powdered flakes. This results in an amorphous layerabsorber layer without crystalline structure.

Some flakes of the invention can be characterized as multilayer thinfilm interference structures in which layers lie in parallel planes suchthat the flakes have first and second parallel planar outer surfaces andan edge thickness perpendicular to the first and second parallel planarouter surfaces. Such flakes are produced to have an aspect ratio of atleast about 2:1, and preferably about 5-15:1 with a narrow particle sizedistribution. The aspect ratio of the flakes is ascertained by takingthe ratio of the longest planar dimension of the first and second outersurfaces to the edge thickness dimension of the flakes.

In order to impart additional durability to the color shifting flakes,an annealing process can be employed to heat treat the flakes at atemperature ranging from about 200-300° C., and preferably from about250-275° C., for a time period ranging from about 10 minutes to about 24hours, and preferably a time period of about 15-60 minutes.

The color shifting pigment flakes of the present invention can beinterspersed within a pigment medium to produce a colorant compositionwhich can be applied to a wide variety of objects or papers. The pigmentflakes added to a medium produces a predetermined optical responsethrough radiation incident on a surface of the solidified medium.Suitable pigment media include various polymeric compositions or organicbinders such as acrylic melamine, urethanes, polyesters, vinyl resins,acrylates, methyl methacrylate, ABS resins, epoxies, styrenes, ink andpaint formulations based on alkyd resins, and mixtures thereof. Thecolor shifting flakes combined with the pigment media produce a colorantcomposition that can be used directly as a paint, ink, or moldableplastic material. The colorant composition can also be utilized as anadditive to conventional paint, ink, or plastic materials.

In addition, the color shifting flakes can be optionally blended withvarious additive materials such as conventional pigment flakes,particles, or dyes of different hues, chroma and brightness to achievethe color characteristics desired. For example, the flakes can be mixedwith other conventional pigments, either of the interference type ornoninterference type, to produce a range of other colors. Thispreblended composition can then be dispersed into a polymeric mediumsuch as a paint, ink, plastic or other polymeric pigment vehicle for usein a conventional manner.

Examples of suitable additive materials that can be combined with thecolor shifting flakes of the invention include non-color shifting highchroma or high reflective platelets which produce unique color effects,such as MgF₂/Al/MgF₂ platelets or SiO₂/Al/SiO₂ platelets. Other suitableadditives that can be mixed with the color shifting flakes includelamellar pigments such as aluminum flakes, graphite flakes, glassflakes, iron oxide, boron nitride, mica flakes, interference based TiO₂coated mica flakes, interference pigments based on multiple coatedplate-like silicatic substrates, metal-dielectric or all-dielectricinterference pigments, and the like; and non-lamellar pigments such asaluminum powder, carbon black, ultramarine blue, cobalt based pigments,organic pigments or dyes, rutile or spinel based inorganic pigments,naturally occurring pigments, inorganic pigments such as titaniumdioxide, talc, china clay, and the like; as well as various mixturesthereof. For example, pigments such as aluminum powder or carbon blackcan be added to control lightness and other color properties.

The color shifting flakes of the present invention are particularlysuited for use in applications where colorants of high chroma anddurability are desired. By using the color shifting flakes in a colorantcomposition, high chroma durable paint or ink can be produced in whichvariable color effects are noticeable to the human eye. The colorshifting flakes of the invention have a wide range of color shiftingproperties, including large shifts in chroma (degree of color purity)and also large shifts in hue (relative color) with a varying angle ofview. Thus, an object colored with a paint containing the color shiftingflakes of the invention will change color depending upon variations inthe viewing angle or the angle of the object relative to the viewingeye.

The color shifting flakes of the invention can be easily andeconomically utilized in paints and inks which can be applied to variousobjects or papers, such as motorized vehicles, currency and securitydocuments, household appliances, architectural structures, flooring,fabrics, sporting goods, electronic packaging/housing, productpackaging, etc. The color shifting flakes can also be utilized informing colored plastic materials, coating compositions, extrusions,electrostatic coatings, glass, and ceramic materials. Because thetitanium-based absorbers used in the pigment flakes are chemically andenvironmentally benign, the pigment flakes are particularly useful inproducts such as cosmetics, toys for children, and fashion apparel suchas in leather goods and cloth goods that are constantly touched.

Generally, the color shifting foils of the invention have anonsymmetrical thin film coating structure, which can correspond to thelayer structures on one side of a reflector in any of the abovedescribed embodiments related to thin film stack flakes. For example, afoil can be formed with repeating dielectric/absorber periods on oneside of a reflector layer such as shown for the flake in FIG. 5. Thefoils can be laminated to various objects or can be formed on a carriersubstrate.

FIG. 9 depicts a coating structure of a color shifting foil 90 formed ona substrate 92, which can be any suitable material such as a flexiblePET web, carrier substrate, or other plastic material. A suitablethickness for substrate 92 is, for example, about 2 to 7 mils. The foil90 includes a reflector layer 94 on substrate 92, a dielectric layer 96on reflector layer 94, and a titanium-based absorber layer 98 ondielectric layer 96. The reflector, dielectric and absorber layers canbe composed of the same materials and can have the same thicknesses asdescribed above for the corresponding layers in flake 10.

The foil 90 can be formed by a web coating process, with the variouslayers as described above sequentially deposited on a web byconventional deposition techniques to form a thin film foil structure.The foil 90 can be formed on a release layer of a web so that the foilcan be subsequently removed and attached to a surface of an object. Thefoil 90 can also be formed on a carrier substrate, which can be a webwithout a release layer.

FIG. 10 depicts a coating structure of a color shifting foil 100 formedon a carrier substrate 102. The foil 100 includes a first titanium-basedabsorber layer 104 on substrate 102, a dielectric layer 106 on absorberlayer 104, and a second titanium-based absorber layer 108 on dielectriclayer 106, but does not include a reflector layer. Such a film structureallows the foil to be transparent to light incident upon the surfacethereof, thereby providing for visual verification or machinereadability of information below foil 100 on carrier substrate 102. Thedielectric and absorber layers of foil 100 can be composed of the samematerials and can have the same thicknesses as described above for thecorresponding layers in flake 10.

The foils of the invention can be used in a hot stamping configurationwhere the thin film stack of the foil is removed from the release layerof a substrate by use of a heat activated adhesive. The adhesive can beeither coated on a surface of the foil opposite from the substrate, orcan be applied in the form of a UV activated adhesive to the surface onwhich the foil will be affixed. Further details of making and usingoptical stacks as hot stamping foils can be found in U.S. Pat. Nos.5,648,165, 5,002,312, 4,930,866, 4,838,648, 4,779,898, and 4,705,300,the disclosures of which are incorporated by reference herein.

FIG. 11 illustrates one embodiment of a foil 110 disposed on a web 112having an optional release layer 114 on which is deposited reflectorlayer 116, dielectric layer 118, and titanium-based absorber layer 120.The foil 110 may be utilized attached to web 112 as a carrier when arelease layer is not employed. Alternatively, foil 110 may be laminatedto a transparent substrate (not shown) via an optional adhesive layer122, such as a transparent adhesive or ultraviolet (UV) curableadhesive, when the release layer is used. The adhesive layer 122 isapplied to absorber layer 120.

FIG. 12 depicts an alternative embodiment in which a foil 130 having thesame thin film layers as foil 110 is disposed on a web 112 havingoptional release layer 114. The foil 130 is formed such thattitanium-based absorber layer 120 is deposited on web 112. The foil 130may be utilized attached to web 112 as a carrier, which is preferablytransparent, when a release layer is not employed. The foil 130 may alsobe attached to a substrate (not shown) when the release layer is used,via an adhesive layer 132 such as a hot stampable adhesive, a pressuresensitive adhesive, a permanent adhesive, and the like. The adhesivelayer 132 is applied to reflector layer 116.

The titanium-containing absorber layer in the pigment flakes of theinvention provides the benefits of having benign chemicalcharacteristics, as well as avoiding metal welding during the flakecoating process. The titanium-based absorbers also avoid theauto-ignition problem of prior titanium coating techniques. In addition,as shown in the examples below, the pigments and foils of the presentinvention are durable toward water, acid, bleach and base. The pigmentsand foils are also stable to ultraviolet radiation (UV) exposure sincenone of the components used in preparing the pigments or foils are knownto be UV sensitive.

The following examples are given to illustrate the present invention,and are not intended to limit the scope of the invention.

EXAMPLES

In order to quantify the color characteristics of a particular object,it is useful to invoke the L*a*b* color coordinate system developed bythe Commission Internationale de l'Eclairage (CIE), which is now used asa standard in the industry in order to precisely describe color values.In this system, L* indicates lightness and a* and b* are thechromaticity coordinates. In some of the examples which follow, thecolor characteristics of a pigment are set forth in terms of L*, chroma(C*) which corresponds to color purity, and hue (h) which corresponds tocolor variation with changing angle.

The L*a*b*color system allows for a comparison of the color differencesbetween two measurements through the parameter ΔE_(ab), which indicatesthe change in color as measured in the L*a*b* color space, such as thecolor difference of two different pigment designs. The numerical valuefor ΔE_(ab) is calculated through the following equation using themeasured L*a*b* values:ΔE _(ab)=[(ΔL*)²+(Δa*)²+(Δb*)²]^(1/2)where the symbol Δ (or D used in some examples below) denotes thedifference in measurements being compared.

Example 1

Theoretical modeling was performed using conventional optical designsoftware to analyze thin film stacks having the following multilayerstructure:

-   -   absorber/SiO₂(4 QW)/Al(800 Å)/SiO₂(4 QW)/absorber. The        dielectric SiO₂ layer optical thickness varied from 4 quarter        waves (QW) at 350 nm to 4 QW at 800 nm. FIGS. 13 and 14 are        graphs showing theoretical chromaticity plots based on the        theoretical modeling of this multilayer structure. In general,        the graphs of FIGS. 13 and 14 show that based on theoretical        modeling, this thin film stack structure with various absorber        materials at varying thicknesses will produce a high chroma.

In particular, FIG. 13 shows the theoretical plots of a*, b*, where thechroma is defined as C*=√{square root over ((a*²+b*²))}, i.e., thechroma value is a vector starting from the origin. The further out fromthe origin, the larger the chroma value. FIG. 13 shows the plots forvarious absorbers, including 55 Å of Cr as an absorber, 90 Å of graphiteas an absorber, and 260 Å of carbon from a carbon arc source as anabsorber. All of the plot curves for each of these absorbers in FIG. 13trace a similar but somewhat different trajectory. FIG. 14 shows thetheoretical plots of a*, b* for various absorbers, including 55 Å of Cras an absorber, 55 Å of Ti as an absorber, and 150 Å of TiN as anabsorber. All of the plot curves for each of these absorbers trace avery similar trajectory. Thus, the chroma values for the films havingabsorber layers of Ti or TiN is similar to the chroma value for the filmwith chromium absorber layers.

Example 2

A color shifting powdered pigment according to the present invention wasproduced by coating a sol-gel based powder material with titaniumnitride. The sol-gel material was composed of particles with thestructure SiO₂/BMF/SiO₂, where the SiO₂ was formed fromtetraethoxysilane (TEOS). The BMF material was formed on a release layerof a web in a roll coater as SiOx/Al/SiOx and removed by dissolving therelease layer. Following removal of the BMF material from the web, thematerial was ground to about 30 μm. The BMF material was then reacted ina stirred reactor with IPA/water, ammonia and TEOS. The TEOS wasreactively decomposed to form SiO₂ onto the surfaces of the SiOx. Thethickness of the SiO₂ can be adjusted as desired to form the appropriatecolor shift. The sol-gel material was then dried and calcined at 500° C.

The sol-gel material in an amount of 240 grams was then loaded into aparticle vacuum coater having vibrating trays which held the sol-gelparticles. The particle vacuum chamber was pumped down to a pressure of2×10⁻⁵ torr. The vibrating trays were turned on prior to the pump-downso that entrapped air would not suddenly erupt and cause powder tostream out into the chamber. Nitrogen gas was then leaked into thechamber at a rate of 20 sccm and the chamber pressure was maintained at2×10⁻³ torr with the addition of argon. One titanium target wasenergized at 7 kW. The titanium reacted with the nitrogen gas formingtitanium nitride on the surface of the sol-gel particles. An in-situcolor monitor indicating the highest level of chroma on the movingparticles was used to determine coating time, which was 110 minutes.Following coating, the chamber was allowed to cool under oxygen at apressure of 2×10⁻³ torr while the coated material was circulated in thetrays. After the chamber temperature had declined to about 35° C. (afterabout 40 min), the chamber was slowly brought up to atmospheric pressurewith atmospheric air. This process produced a color shifting powderedpigment (TiN pigment) exhibiting high chroma and good stability towardwater, acid, base, bleach, and UV exposure.

An 8 mil film sample was prepared using the TiN pigment in a paintvehicle (Du Pont Refinish 150K) with an 8 pass wet film applicator(Doctor Blade device). One part pigment to 9 parts paint vehicle wasused. The film sample exhibited a blue-to-red color shift at differingangles of incident light or viewing.

Table 1 below lists the color data generated for a standard black filmand the above film sample having the TiN pigment, including the valuesmeasured for L*, a*, b*, C* and h, along with the change in each ofthese values (Δ) between the standard black film and the film sample.The illuminant/observer conditions used in generating the data in Table1 included a light source of Δ65 10 deg (6500K black body light sourceat 10 degrees) for illuminating the films.

TABLE 1 Color Value Black Film Sample Δ L* 0.05 42.55 42.51 a* 0.0923.49 23.40 b* −0.07 −36.75 −36.69 C* 0.11 43.62 43.51 h  322.99 302.59−0.78

The numerical value for ΔE_(ab) indicating the change in color wascalculated from the L*a*b* values in Table 1 to be 60.83.

The TiN pigment was also drawn down into a pigment vehicle (UnionCarbide VYHH Vinyl Resin, a vinyl chloride-vinyl acetate copolymer) toform a film in which the pigment to vehicle ratio by weight was 1:4.5.This film was characterized as to durability in water, acid, base, andbleach by measuring film samples on a DataColor spectrophotometer withD65 illumination (6500K black body light source) and 10 degreesobservation. The film samples were measured after 24 hours of immersionin water at 140° F.; after 30 minutes of immersion in 2% by volumesulfuric acid (H₂SO₄); after 10 minutes of immersion in 2% by weightsodium hydroxide (NaOH); and after 10 minutes of immersion in 20% byvolume Clorox bleach at room temperature. The change in color datameasurements for these film samples with respect to the samemeasurements of the film sample prior to durability testing aresummarized below in Table 2.

TABLE 2 Test Sample ΔL* Δa* Δb* ΔC* Δh ΔE 1 (Water) −0.55 −2.34 0.29−1.38 −1.91 2.42 2 (Acid) 0.26 0.12 −0.76 0.72 −0.27 0.81 3 (Base) −0.39−1.75 1.71 −2.34 −0.72 2.48 4 Bleach) −0.14 −0.74 0.17 −0.51 −0.57 0.78

The excellent durability for the four film samples listed in Table 2 isevident by the small changes, on average, in ΔE. Large changes in ΔEwould be on the order of 20-30, whereas small changes are consideredless than 10 and more preferably less than 5.

Example 3

A color shifting powdered pigment according to the present invention wasproduced by coating a sol-gel based powder material with titanium andcarbon. The sol-gel material was composed of particles with thestructure SiO₂/BMF/SiO₂, which were produced by the same procedure asdescribed above for the sol-gel material in Example 1.

The sol-gel material in an amount of 380 grams was then loaded into aparticle vacuum coater having vibrating trays which held the sol-gelparticles. The particle vacuum chamber was pumped down to a pressure of2×10⁻⁵ torr. The vibrating trays were turned on prior to the pump-downso that entrapped air would not suddenly erupt and cause powder tostream out into the chamber. The chamber pressure was maintained at2×10⁻³ torr with the addition of argon. Two titanium targets wereenergized at 5 kW and one carbon target was powered at 6 kW. The sol-gelparticles moved consecutively under the two titanium targets and thenunder the carbon target to form alternating sublayers of titanium andcarbon on the particles. The coating time was 130 minutes.

Following coating, the chamber was allowed to cool under oxygen at apressure of 2×10⁻³ torr while the coated material was circulated in thetrays. After the chamber temperature had declined, the chamber wasslowly brought up to atmospheric pressure. The above process produced acolor shifting powdered pigment (Ti-C pigment) exhibiting high chromaand good stability toward water, acid, base, bleach, and UV exposure.

An 8 mil film was prepared using the Ti-C pigment in the same paintvehicle and in the same manner as described above in Example 1, with onepart pigment to 9 parts paint vehicle being used. The film exhibited agreen-to-blue color shift at differing angles of incident light orviewing.

Table 3 below lists the color data generated for a standard black filmand the above film sample having the Ti-C pigment, including the valuesmeasured for L*, a*, b*, C* and h, along with the change in each ofthese values (Δ) between the standard black film and the film sample.The illuminant/observer conditions used in generating the data in Table3 included a light source of Δ65 10 deg for illuminating the films.

TABLE 3 Color Value Black Film Sample Δ L* 0.05 60.03 59.99 a* 0.09−42.95 −43.04 b* −0.07 −2.35 −2.29 C* 0.11 43.02 42.91 h  322.99 183.14−4.10

The numerical value for ΔE_(ab) indicating the change in color wascalculated from the L*a*b* values in Table 3 to be 73.87.

Example 4

A color shifting powdered pigment according to the present invention wasproduced by coating a sol-gel based powder material with titaniumnitride. Each of the pigment flakes had the coating structureTiN_(x)/SiO₂(sol-gel)/Al(sputtered)/SiO₂(sol-gel)/TiN_(x). The pigmentwas produced by a similar procedure as described above for the sol-gelmaterial in Example 1. The pigment was drawn down into a pigment vehicle(Union Carbide VYHH Vinyl Resin) to form a film in which the pigment tovehicle ratio by weight was 1:5. This film was characterized as todurability in water, acid, base, and bleach by measuring film samples ona DataColor spectrophotometer with D65 illumination and 10 degreesobservation. The film samples were measured after 24 hours of immersionin water at 140° F.; after 30 minutes of immersion in 2% by volumesulfuric acid (H₂SO₄); after 10 minutes of immersion in 2% by weightsodium hydroxide (NaOH); and after 10 minutes of immersion in 20% byvolume Clorox bleach at room temperature. Color data measurements forthese film samples are summarized below in Table 4, which showsexcellent durability for the films.

TABLE 4 Test Sample ΔL* Δa* Δb* ΔC* ΔH* ΔE* 1 (Water) −1.85 −2.12 0.47−1.48 −1.59 2.85 2 (Acid) 0.45 0.25 −0.51 0.57 −0.06 0.73 3 (Base) 0.16−1.34 1.39 −1.88 −0.43 1.94 4 (Bleach) 0.53 −1.05 0.77 −1.20 −0.50 1.40

Example 5

Color shifting films having a TiN_(x) absorber layer were produced bydirect microwave PECVD according to the following procedure. Glasssubstrates (1″×1″) were pre-coated with an 80 nm thick aluminum layer,and an SiO₂ dielectric film having an optical thickness of 4 quarterwaves (QW) at 550 nm. A plasma reactor, which can be described as adirect microwave PECVD device, was utilized to deposit the TiN_(x)absorbing layer on the pre-coated substrates.

The pre-coated substrates (samples 1-15) were each separately placed ona substrate holder, which had a 100 mm diameter, at about 4 inches froma fused silica (dielectric) window placed between a symmetric plasmacoupler and a reactor chamber. The titanium-containing gas, TiCl₄, wasselected because it is inexpensive and a liquid at room temperature witha high vapor pressure. The TiCl₄ gas flow was adjusted with a vapor flowcontroller and was introduced in the vicinity of the substrate holderusing a gas distributor. The other gases, Ar, N₂ and H₂, were introducedinto the reactor chamber close to the window. The microwave power wasfixed at 600 watts. The substrate temperature, flow of gases, totalpressure, and deposition time were varied to obtain the best opticalproperties.

Table 5 below summarizes the process conditions for the deposition ofthe absorber layer on each of substrate samples 1-15 by direct microwavedischarge.

TABLE 5 Sam- TiCl₄ Ar H₂ N₂ Pressure Temp. ple [sccm] [sccm] [sccm][sccm] [Torr] [° C.] Time 1 6 25 70 15 100 200  3 min 2 6 25 70 15 100200  5 min 3 6 25 70 15 100 200  2 min 4 6 25 70 15 100 300  2 min 5 625 70 15 80 200  2 min 6 6 25 70 20 80 200  1 min 45 sec 7 6 25 70 15100 200  4 min 8 6 25 70 15 100 200  2 min 30 sec 9 6 25 70 15 100 200 1 min 45 sec 10 6 25 70 15 100 200  1 min 10 sec 11 6 25 70 15 100 20045 sec 12 6 25 70 15 100 300 45 sec 13 6 25 70 15 100 Room  1 min 14 625 70 15 100 350  1 min 05 sec 15 6 25 70 15 100 400  1 min

Table 6 below gives the color data measurements for the best opticalperformance obtained among samples 1-15.

TABLE 6 Sample L* C* H 9 48.61 40.33 348.17 14 77.38 40.19 104.79

Differing process conditions and absorber layer thicknesses can changethe final color shifting of the film design. For example, sample 9produced a red-to-green color shift while sample 14 produced agreen-to-blue color shift, even though the original pre-coatedsubstrates had the same optical design.

FIG. 15 is a graph showing the transmittance of the absorber layer as afunction of wavelength for the color shifting film corresponding tosample 14 before and after being placed in water at 60° C. for 24 hours.This graph shows that for some conditions, the absorber layers are verystable against oxidation.

It should be noted that similar results can be obtained if the plasmacoupler and the fused silica window are located over vibrating trayswhere flakes with an equivalent optical design (SiO₂(4QW)/Al(80nm)/SiO₂(4QW)) are flowing such as described previously.

Example 6

Color shifting films having TiN, TiC, or TiN_(x)C_(z) absorber layerswere produced by RF PECVD according to the following procedure. Glasssubstrates such as those described in Example 3 were pre-coated with an80 nm thick aluminum layer, and an SiO₂ dielectric film having anoptical thickness of 4 quarter waves (QW) at 550 nm. A plasma reactor,which can be described as an RF powered downstream (Ds) PECVD device,was utilized to deposit the TiN, TiC, or TiN_(x)C_(z) absorbing layerson the pre-coated substrates.

The pre-coated substrates (samples 16-30) were each separately placed ona substrate holder, which had a 100 mm diameter, at about 4 inches fromthe end of a dielectric chamber. The process gases Ar, N₂, NH₃, CH₄ andH₂ were introduced into the plasma zone in the dielectric chamber, andthe TiCl₄ gas was introduced in the vicinity of the substrate holderusing a gas distributor. The pressure was fixed at 100 mTorr. Thesubstrate temperature, flow of gases, RF power, and deposition time werevaried to obtain the best optical properties.

Table 7 below summarizes the process conditions for the deposition ofthe absorber layer on each of substrate samples 16-30 by RF downstreamplasma.

TABLE 7 Rf TiCl₄ Ar H₂ N₂ power Temp. Time Sample [sccm] [sccm] [sccm][sccm] [W] [° C.] [sec] 16 5 25 90 35 100 400 60 (TiN) 17 5 25 90 35 150400 50 (TiN) 18 5 25 90 35 400  25 60 (TiN) 19 5 25 90 35 300 400 60(TiN) 20 5 25 90 35 300 300 60 (TiN) 21 5 25 90 35 300 200 135  (TiN) RfTiCl₄ Ar H₂ CH₄ power Temp. Time [sccm] [sccm] [sccm] [sccm] [W] [° C.][sec] 22 5 25 90 35 300 200 135  (TiC) 23 5 25 25 10 300 200 120  (TiC)24 6 25 25 25 300 400 210  (TiC) Rf TiCl₄ Ar N₂ CH₄ power Temp. Time[sccm] [sccm] [sccm] [sccm] [W] [° C.] [sec] 25 6 25 35 25 300 300 195 (TiCN) 26 6 25 35 25 300 300 60 (TiCN) 27 6 25 35 25 300 200 75 (TiCN)Rf TiCl₄ Ar N₂ NH₃ power Temp. Time [sccm] [sccm] [sccm] [sccm] [W] [°C.] [sec] 28 6 25 No 80 300 200 125  (TiN) 29 6 50 No 60 300 200 180 (TiN) 30 6 25 No 100  300 200 240  (TiN)

Table 8 below lists the color data measurements for the best opticalperformance obtained among samples 16-30, and for a comparative samplewhich had a chromium absorber layer evaporated onto the same type ofpre-coated substrate as used in samples 16-30

TABLE 8 Sample L* C* H Cr (Evap.) 80.41 69.12 137.92 17 78.68 82.98143.63 18 77.56 81.44 144.66 16 79.79 78.22 133.26 19 80.04 77.46 136.3521 83.68 68.6 131.28 20 88.96 36.47 144.16

As shown in Table 8, some process conditions produced films (samples16-19) with higher chromas than that of the sample with the chromiumabsorber layer. FIG. 16 is a graph showing the transmittance of theabsorber layer for the color shifting film corresponding to sample 17both before and after being placed in water at 60° C. for 24 hours. Thegraph of FIG. 16 shows that this layer had good stability againstoxidation. The film of sample 17 produced a green-to-blue color shift.

It should be noted that similar results can be obtained if an RF plasmasource is located over vibrating trays where flakes with an equivalentoptical design (SiO₂(4QW)/Al(80 nm)/SiO₂(4QW) ) are flowing such asdescribed previously.

Example 7

A color shifting film having a Ti absorber layer was formed on ahardcoated polyethylene terephthalate (PET) web (Avery Hardcoated PET)with the following coating design:

-   -   Ti(150 Å)/MgF₂(4QW at 700 nm)/Al(850 Å)/MgF₂(4QW at 700        nm)/Ti(150 Å).

FIG. 17 is a graph showing reflectance as a function of wavelength forthe front side of this foil film. FIG. 18 is a graph showing reflectanceas a function of wavelength for the back side of this foil film. Thesegraphs show that the Ti absorber in the film provides good absorptionfrom about 550 nm to about 600 nm in the visible spectrum. It should benoted that both sides of the film have essentially the same opticalproperties as is required for high chroma pigments based on thetechnologies disclosed herein.

Example 8

A color shifting foil having a Ti absorber layer was formed on ahardcoated PET web (Avery Hardcoated PET). The foil had the coatingstructure:

-   -   Ti(100 Å)/MgF₂(4QW at 632 nm)/Al(opaque)/MgF₂(4QW at 632        nm)/Ti(100 Å).

Table 9 below lists the color data generated for various samples of thefoil, including the X, Y, Z tristimulus values and the values measuredfor L*, a*, b*, C*, and h, as well as the illuminant/observer conditionsused in generating the data in Table 9. The data was generated using aZeiss gonoiospectrophotometer, with a light source of Δ65 at 10 degreesobservation used to illuminate the samples. The illumination angle wasfrom 180°, the observer angle (view) was from 0°, and the angle betweenillumination and observer (Diff.) was 10°.

TABLE 9 Illum. Sample angle View Diff. Filter X Y Z L* a* b* C* h 1 25145 10 1 65.06 60.59 238.22 82.16 17.92 −91.64 93.37 281.07 2 30 140 101 52.86 60.98 196.55 82.37 −12.48 −75.08 76.11 260.56 3 35 135 10 145.26 65.50 149.51 84.74 −43.45 −49.67 65.99 228.82 4 40 130 10 1 40.8969.68 103.81 86.84 −65.53 −20.47 68.65 197.35 5 45 125 10 1 40.02 72.8867.39 88.39 −74.90 8.73 75.40 173.35 6 50 120 10 1 40.79 73.36 41.5288.62 −73.50 34.66 81.26 154.76 7 55 115 10 1 42.72 72.44 25.64 88.18−65.75 55.53 86.06 139.82 8 60 110 10 1 44.50 70.09 16.58 87.04 −55.5670.35 89.64 128.30 9 65 105 10 1 45.91 67.14 11.87 85.57 −45.18 79.1291.11 119.73 10 70 100 10 1 47.03 64.64 10.04 84.30 −36.52 82.13 89.88113.97 11 75 95 10 1 47.59 62.37 9.41 83.11 −29.83 82.03 87.28 109.98

FIGS. 19A and 19B make up an L*a*b* diagram which plots the colortrajectory and chromaticity of the foil of example 8 at the eleven (11)different angles of viewing shown in Table 9 under specular conditions.It can be seen in referring to FIGS. 19A and 19B that the color atnormal viewing condition is yellow. As the angle of viewing becomeslarger relative to 90° to the plane of the sample, the color changesfrom yellow-to-green-to-blue.

Example 9

Color shifting pigments were produced in which the pigment flakes hadfollowing coating structures:

-   -   Sample 1) Ti/MgF₂(4QW at 550 nm)/Al/MgF₂ (4QW at 550 nm)/Ti; and    -   Sample 2) Cr/MgF₂(6QW at 550 nm)/Al/MgF₂ (6QW at 550 nm)/Cr.

The pigments were drawn down into a pigment vehicle to form paintedpanels. FIG. 20 is a chromaticity graph showing the measured colortrajectories for the painted panels. The color trajectories shown in thegraph of FIG. 20 were generated using a Zeiss gonoiospectrophotometer.The graph of FIG. 20 shows the color trajectory for a painted panelhaving the sample 1 pigment, which had a green-to-blue color shift, andthe color trajectory for a painted panel having the sample 1 pigmentovercoated with a clear coat (cc) on the painted panel. The graph ofFIG. 20 also shows the color trajectory for a painted panel having thesample 2 pigment, which had a green-to-purple color shift. The graph ofFIG. 20 shows that high chroma that can be achieved when Ti is used asan absorber layer (sample 1), especially in pigment form, with thesample 1 pigment producing a greater chroma than the sample 2(conventional) pigment having a Cr absorber.

Example 10

Various color shifting pigments with titanium, carbon, or chromiumabsorbers were produced by depositing multilayer thin films on a web byelectron beam, removing the films from the web in the form of flakes,and grinding the flakes to form the final pigment. The pigment flakeshad following coating structure: absorber/MgF₂/Al/MgF₂/absorber.

Various pigment samples were drawn down into a pigment vehicle to formfilms which were characterized as to durability in water, acid, base,and bleach. The change in color data measurements (ΔE) for these filmsamples with respect to the same measurements of the film sample priorto durability testing are summarized below in Table 10. The data inTable 10 compares the effect of absorber material type on durabilitywith everything else being equal.

TABLE 10 ΔE ΔE ΔE ΔE Sample Absorber Chroma Hue acid base bleach water1, 2 C 39.09 125.82 0.81 2.34 2.68 1.00 3, 4 C 34.09 72.68 0.18 3.741.73 0.19 5 C 38.32 169.79 0.38 1.98 0.84 4.31 6, 7 Cr 33.56 39.09 0.4113.79 5.36 7.16 8, 9, 10 Cr 57.87 120.62 0.52 6.27 8.15 1.20 11, 12 Cr59.12 124.23 0.49 3.05 2.99 1.81 13, 14 Cr 55.06 97.89 0.47 15.96 8.915.23 15, 16 Ti 27.89 348.30 3.16 14.43 0.95 5.92 17, 18, 19 Ti 53.02137.33 1.42 5.13 6.19 2.84 20, 21 Ti 42.93 125.16 1.50 3.99 4.73 3.4222, 23 Ti 36.23 97.79 0.73 13.24 3.13 3.84 24, 25 Ti 39.88 106.87 0.844.00 5.58 3.69

Example 11

Various color shifting pigments with titanium absorbers at differingthicknesses were produced by depositing multilayer thin films on ahardcoated web, removing the films from the web in the form of flakes,and grinding the flakes to form the final pigment. The pigment flakesfor each of the various pigments had the following coating structures:

-   -   1) Ti(50 Å)/MgF₂(4QW at 632 nm)/Al(1000 Å)/MgF₂(4QW at 632        nm)/Ti(50 Å);    -   2) Ti(75 Å)/MgF₂(4QW at 632 nm)/Al(1000 Å)/MgF₂(4QW at 632        nm)/Ti(75 Å);    -   3) Ti(100 Å)/MgF₂(4QW at 632 nm)/Al(1000 Å)/MgF₂(4QW at 632        nm)/Ti(100 Å);    -   4) Ti(125 Å)/MgF₂(4QW at 632 nm)/Al(1000 Å)/MgF₂(4QW at 632        nm)/Ti(125 Å);    -   5) Ti(150 Å)/mgF₂(4QW at 632 nm)/Al(1000 Å)/MgF₂(4QW at 632        nm)/Ti(150 Å).

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

1. A color shifting pigment flake, comprising: a core layer; a firsttitanium-containing absorber layer substantially surrounding the corelayer; a dielectric layer substantially surrounding the firsttitanium-containing absorber layer; and a second titanium-containingabsorber layer substantially surrounding the dielectric layer; whereinthe pigment flake exhibits a discrete color shift such that the pigmentflake has a first color at a first angle of incident light or viewingand a second color different from the first color at a second angle ofincident light or viewing.
 2. The pigment flake of claim 1, wherein thecore layer is composed of mica or glass.
 3. The pigment flake of claim1, wherein the dielectric layer is composed of silicon dioxide ortitanium dioxide.
 4. The pigment flake of claim 3, wherein thedielectric layer is formed by a sol-gel process.
 5. The pigment flake ofclaim 1, wherein the first and second titanium-containing absorberlayers comprise a material selected from the group consisting ofelemental titanium, titanium-based compounds, titanium-based alloys, andcombinations thereof.
 6. The pigment flake of claim 1, wherein the firstand second titanium-containing absorber layers comprise an absorbingmaterial selected from the group consisting of titanium, titaniumnitride, titanium oxynitride, titanium carbide, titanium mixed withtungsten, titanium mixed with silicon, titanium mixed with niobium, andcombinations thereof.
 7. A color shifting pigment flake, comprising: afirst titanium-containing absorber layer; a dielectric layer on thefirst absorber layer; and a second titanium-containing absorber layer onthe dielectric layer; wherein the pigment flake exhibits a discretecolor shift such that the pigment flake has a first color at a firstangle of incident light or viewing and a second color different from thefirst color at a second angle of incident light or viewing.
 8. Thepigment flake of claim 7, wherein the first and secondtitanium-containing absorber layers comprise a material selected fromthe group consisting of elemental titanium, titanium-based compounds,titanium-based alloys, and combinations thereof.
 9. The pigment flake ofclaim 7, wherein the first and second titanium-containing absorberlayers comprise an absorbing material selected from the group consistingof titanium, titanium nitride, titanium oxynitride, titanium carbide,titanium oxynitride carbide, titanium silicide, titanium boride,titanium mixed with carbon, titanium mixed with tungsten, titanium mixedwith silicon, titanium mixed with niobium, and combinations thereof. 10.The pigment flake of claim 7, wherein the first and secondtitanium-containing absorber layers from a continuous coating layeraround the dielectric layer.